The present invention generally relates to multistage recrystallization, and more particularly to a multistage recrystallization method and apparatus for superpurifying crystallizable substances to purify them to extremely high levels, typically on the order of 99.999 percent purity. Reflux ratio conditions are controlled closely, referably in association with the establishment and maintenance of steady state conditions in each stage of the system, thereby facilitating generally automatic control of reflux ratio conditions by closely monitoring and metering selected material transfers, inputs and/or outputs for each stage of the system.
Purification of crystallizable materials by way of recrystallization techniques has been known and practiced for many years. Generally speaking, it is extremely difficult to remove trace impurities from crystallizable materials by recrystallization from a saturated solution therefor, or by other techniques including fractional crystallization and fractional freezing procedures. Included are zone-refining procedures which typically would increase product purity by adding recrystallization stages, but this would be done at the expense of the percentage yield of product that is formed at the last or purest recrystallization stage. For example, it is well known to use fractional crystallization in separating radium chloride from barium chloride by a classic separation procedure that utilizes a complicated grid pattern of many dozens of separate evaporation dishes or crystallizers. Under this procedure, the crystal crop of each crystallizer is transferred to the adjacent crystallizer in one direction, while the mother liquor is transferred in the opposite direction. Although increasing concentrations of radium chloride follow movement of the crystal crop from one crystallizer to the next, a very large number of crystallizers are required, and operating costs for this type of system are very high.
Generally speaking, currently available applications for fractional crystallization have recognized the desirability of operating at an optimum reflux ratio while not necessarily effectively providing suitable specifics for implementing this desirable approach.
The multistage recrystallization art includes multistep recrystallization systems for separating fluid material having two or more different components. For example, U.S. Pat. No. Re. 32,241 describes having a component crystallize on a cooled surface as material containing the component flows theredown. The process can be used to crystallize a component from its melt. In this patent, a liquid of a given stage is used to wash the crystals formed in that stage before those crystals are transferred to the next purer stage. This wash liquid is the material that is in this stage at a time immediately preceding crystallization therefrom. In an embodiment of that patent, the crystals are transferred to the next purer stage as soon as appropriate valves cause the liquid of the next purer stage to contact and dissolve the crystals. The wash liquid, which is used to wash the crystal crop of a given stage, is the purest part of the liquid present in that given stage before any material is crystallized from it.
Publications such as U.S. Pat. No. 4,666,456 describe what are essentially a single stage crystallization systems. This particular patent shows a fractional freezing process which includes continuous partial crystallization of a compound from a liquid mixture in which the mixture is fed through a cascade of a plurality of cooling sections. These cooling sections are connected in series, and the temperature of each subsequent one of the cascading cooling sections is lower than that of the preceding one. A percentage of the mixture freezes out in each stage. Mother liquor is transferred between two adjacent stages.
It has been found that, by proceeding in accordance with the present invention, it is possible to achieve superpurification of crystallizable substances by removal of certain impurities therefrom through a multistage fractional freezing recrystallization procedure that achieves purity levels on the order of 99.999 percent with extremely high yields that are exceptional for multistage recrystallization procedures. Included is an effective manner of controlling reflux ratio conditions while achieving and maintaining steady state operating conditions.
In summary, the present invention is a method and apparatus for superpurifying crystallizable substances by a multistage recrystallization procedure which includes measures to control reflux ratio conditions by providing metering procedures that control quantities of crystals and mother liquor reflux materials that are transferred according to the method and apparatus. In each stage, a crystallizable substance is frozen and recrystallized, and the crystals are separated from the mother liquor. A controlled quantity of the mother liquor is metered. In the least pure stage, this metered mother liquor provides a controlled quantity of byproduct. This metered mother liquor of the other stages is transferred to the crystals that had been separated from the next less pure stage in order to thereby wash same. Crystals collected in the purest stage provide a predetermined quantity of crystal product. A predetermined quantity of crystallizable material is fed to the least pure stage and another cycle is run. If desired, additional, similar stages may be provided in order to concentrate the byproduct so as to form a relatively large percentage of purified material and a smaller percentage of very impure material. This purified material can be used as feed material to the multistage recrystallization system.
It is a general object of the present invention to provide an improved method and apparatus for superpurifying crystallizable substances.
Another object of the present invention is to provide an improved superpurification system that utilizes multistage recrystallization that effectively, efficiently and very closely controls reflux ratio conditions.
Another object of the present invention is to provide an improvement in multistage recrystallization that can achieve and then maintain steady state conditions.
Another object of this invention is to provide improved superpurification of crystallizable substances by generally automatically metering inputs and outputs from the system and between stages of the system which maintain a gradient of differing purity levels.
Another object of this invention is to provide an improved multistage recrystallization system which achieves extremely high purification together with particularly high yields
Another object of the present invention is to provide an improved apparatus and method for superpurification of crystallizable substances which includes the use of arrangements whereby a wash reflux from one stage of a multistage recrystallization system is used to displace residual mother liquor from crystals formed in a stage that is immediately adjacent to and that operates a lower purity levels than the stage from which the wash reflux originates.
Another object of the present invention is to provide a multistage recrystallization system wherein especially efficient and thorough separation of a minor ingredient from a major ingredient is effected.
Another object of this invention is to provide an improved apparatus and method that utilizes multiple stages and involves forming a new crystal crop in each stage.
Another object of this invention is to provide an improved multistage fractional crystallization system having a controlled reflux ratio such that the proper reflux ratio can be used to separate a minor ingredient having a given segregation coefficient.
Another object of the present invention is to provide an improved apparatus and method wherein reflux leaving a given stage of a multistage system is as impure as possible and such that the crystal material leaving that stage is as free of its mother liquor as possible in order to thereby maximize the separation efficiency of each stage.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.